Inefficiency of electrical machines comes primarily from electromagnetic losses (joule losses, iron losses, . . . ) in which energy is dissipated as heat. In permanent magnet generators, for example, these losses mainly come from copper losses in stator conductors, resulting in high temperature in the coils of the electric machine and especially in the end-windings that extend beyond the outer edges of the stator and are surrounded by air. In the coils and in the end windings joule losses are transformed to thermal energy causing heating up the stator. The joule losses are increasing with the length of the coils and of the end-windings. The stator winding temperature must be kept low enough to avoid damaging the insulation lifetime, so an efficient cooling system is of prime importance.
It is therefore important to minimize the winding overhang length in order to increase the machine efficiency and reduce the cooling system size.
For a large armature such as the rotor or stator of a wind turbine, the coil windings are generally quite thick and heavy owing to the physical dimensions of the generator and the high currents induced. The windings can be made of thick wire such as a multi-stranded wire, which is then wound onto the armature, usually the stator. To this end, the armature is usually made with multiple parallel slots arranged axially along the outside for accommodating the windings. Instead of wrapping the wire onto the armature, pre-shaped windings can be formed and inserted or ‘dropped into’ the slots of the armature. Such a pre-shaped winding generally comprises a closed loop comprising a “go” section and a “return” section held in two neighbouring stator or rotor slots. A coil comprises a plurality of such windings connected in series or parallel, and the connection is usually made at one end of the armature where the windings extend beyond the ends of the slots. As will be known to the skilled person, successive windings of a coil can be connected by allowing a strand of a multi-stranded wire to extend from one winding of a coil to the next winding of that coil, or by connecting windings of a coil to a bus bar arranged circumferentially about the stator, or in any other appropriate manner.
A multi-phase generator has the same number of coils as phases. Here, the windings are placed in the slots such that the slots for the “go” and “return” sections of one particular winding of a coil enclose or flank a number of slots for the “go” and “return” sections of the remaining coils. The windings of the different coils must overlap in some way at the stator ends.
Therefore different windings types are provided with different end sections designs. A first winding type is a straightforward closed loop, and the end section of this winding type comprises a 180° fold. A second winding type has an end section that does not extend as far beyond the stator end as the first winding type, but makes an approximately 45° tilt before being folded back on itself. A third winding type has an end section that also does not extend as far beyond the armature end as the first winding type and makes an approximately 90° tilt before being folded back on itself. These different end-sections or overhangs allow the windings to be placed into the armature slots in a straightforward manner.
Further, it is also known that the first and the second end sections are provided with 90° tilts and the third end section comprises no tilt.
Both of these end section designs which allowing an uncomplicated winding assembly result in an increasing overhang length that results in an undesirable decrease of the efficiency of the machine.